1. Field of the Invention
This invention relates to a novel organic acid salt of melamine and a thermosetting or a photocurable and thermosetting coating composition using the organic acid salt of melamine, and more particularly to a thermosetting or a photocurable and thermosetting coating composition serving advantageously as a solder resist which particularly excels in resistance to the action of electroless gold plating.
2. Description of the Prior Art
Owing to the recent rapid advance of semiconductor parts, the electronic devices have been showing the trend toward attaining reductions in size and weight, embodying improvements of performance, and undergoing diversifications of function and, as a consequence, the printed circuit boards have been tending toward increased density of printed circuits and adoption of the technique of mounting component elements on the surface of board (surface mount technology). For the purpose of coping with this densification of printed circuits and, in this connection, improving the reliability of the wiring laid between a pad disposed inside a printed circuit board and the component elements mounted on the surface of the printed circuit board, increasing specifications have been setting out the requirement that the pad be subjected to the electroless gold plating treatment. The thermosetting or the photocurable and thermosetting liquid solder resist commonly used, however, incurs the problem that, while the solder resist is being thermally cured, the resist is caused by the oxidation of a copper foil in the printed circuit board to lose the fast adhesiveness thereof to the copper foil and the part of the resist which borders on the site of the electroless gold plating inevitably peels off. Further, the thermal curing treatment entails the problem of so-called copper discoloration, namely the phenomenon that the copper foil is tarnished and impaired in appearance by the oxidation which occurs in the process of thermal curing.
U.S. Pat. No. 5,100,767 issued to Yanagawa et al. on Mar. 31, 1992 discloses a photosensitive resin composition which contains a carboxyl group-containing epoxy acrylate, tris(2,3-epoxypropyl)isocyanurate, and a compound having a melamine skeleton. In this composition, the carboxyl group of the photosensitive resin gradually reacts with the compound having the melamine skeleton. When they are contained as mixed in the composition, the composition encounters problems with degradation of the stability with time and the developing property. When these components are supplied to the end user as two separate liquids and they are mixed by the end user in preparing the composition for use, this composition encounters problems with stability of viscosity of the produced mixture. Further, the basicity of the compound having the melamine skeleton accelerates the reaction of the carboxyl group of the photosensitive resin with the epoxy resin. When this composition is used as a photocurable and thermosetting liquid solder resist, therefore, it entails the problem that the freedom with which the management of drying is attained in the process of predrying of the resist (the condition of drying for allowing the development) is abridged.